1. Field of the Invention
The present invention relates generally to speech coding and, more particularly, to recovery of erased voice frames during speech decoding.
2. Related Art
From time immemorial, it has been desirable to communicate between a speaker at one point and a listener at another point. Hence, the invention of various telecommunication systems. The audible range (i.e. frequency) that can be transmitted and faithfully reproduced depends on the medium of transmission and other factors. Generally, a speech signal can be band-limited to about 10 kHz without affecting its perception. However, in telecommunications, the speech signal bandwidth is usually limited much more severely. For instance, the telephone network limits the bandwidth of the speech signal to between 300 Hz to 3400 Hz, which is known in the art as the “narrowband”. Such band-limitation results in the characteristic sound of telephone speech. Both the lower limit at 300 Hz and the upper limit at 3400 Hz affect the speech quality.
In most digital speech coders, the speech signal is sampled at 8 kHz, resulting in a maximum signal bandwidth of 4 kHz. In practice, however, the signal is usually band-limited to about 3600 Hz at the high-end. At the low-end, the cut-off frequency is usually between 50 Hz and 200 Hz. The narrowband speech signal, which requires a sampling frequency of 8 kb/s, provides a speech quality referred to as toll quality. Although this toll quality is sufficient for telephone communications, for emerging applications such as teleconferencing, multimedia services and high-definition television, an improved quality is necessary.
The communications quality can be improved for such applications by increasing the bandwidth. For example, by increasing the sampling frequency to 16 kHz, a wider bandwidth, ranging from 50 Hz to about 7000 Hz can be accommodated. This bandwidth range is referred to as the “wideband”. Extending the lower frequency range to 50 Hz increases naturalness, presence and comfort. At the other end of the spectrum, extending the higher frequency range to 7000 Hz increases intelligibility and makes it easier to differentiate between fricative sounds.
The frame may be lost because of communication channel problems that results in a bitstream or a bit package of the coded speech being lost or destroyed. When this happens, the decoder must try to recover the speech from available information in order to minimize the impact on the perceptual quality of speech being reproduced.
Pitch lag is one of the most important parameters for voiced speech, because the perceptual quality is very sensitive to pitch lag. To maintain good perceptual quality, it is important to properly recover the pitch track at the decoder. Thus, a traditional practice is that if the current voiced frame bitstream is lost, pitch lag is copied from the previous frame and the periodic signal is constructed in terms of the estimated pitch track. However, if the next frame is properly received, there is a potential for quality impact because of discontinuity introduced by the previously lost frame.
The present invention addresses the impact in perceptual quality due to discontinuities produced by lost frames.